Linear ethylene polymers such as low density polyethylenes (LLDPE) are widely used in the extrusion of films because of their superior properties compared to high pressure polyethylene (LDPE). However, there are persistent problems in the commercial exploitation of LLDPE resins due to difficulties in extrusion. For example, high back pressure and torque due to extremely high shear viscosity of the linear ethylene polymer are encountered. Futhermore, the extruded films exhibit surface distortion related to melt fracture. Accordingly, much effort has been devoted to finding additives, modifying extrusion conditions and changing die materials in order to alleviate some of the problems. For example, U.S. Pat. No. 3,125,547, which is incorporated by reference, discloses the use of a hexafluoropropylene-vinylidene fluoride copolymer as a film processing aid and broadly suggests a variety of other fluorinated polymers for the same purpose.
Films made from mixtures of polyethylene and at least one percent, but preferably somewhat more, of polyacetal resins, are disclosed in U.S. Pat. No. 4,214,034. The polyacetal resin is used to provide thermal insulation and its value as a processing aid or for any purpose at levels below one weight percent is not disclosed.
This invention relates to the use of polyacetal polymer in small amounts to improve the extrusion characteristics of linear ethylene polymers by reducing melt fracture and head pressure and extruder torque.